The present invention relates to removal of minor amounts of formic acid from acetic acid.
Acetic acid, one of the more important acids of commerce, is produced commercially in large quantities by the liquid phase oxidation of low-molecular-weight hydrocarbons such as butane and by the carbonylation of methanol or an ester, ether or halide derivative thereof, using a catalyst system formed on mixing of a rhodium or iridium compound and a halogen component such as hydrogen iodide or methyl iodide. The acetic acid produced by these processes is of high quality but it sometimes contains small amounts of formic acid. Certain commercial specifications require that the formic acid content in acetic acid should desirably be less than 500 ppm, i.e., 500 parts per million parts of acetic acid, but in some instances the acceptable limit for formic acid is specified as &lt;100 ppm.
Many techniques have been used to decrease the formic acid contained in acetic acid. For example, acetic acid containing this contaminant has been treated according to U.S. Pat. No. 2,656,379 by passing it over activated alumina at temperatures above 260.degree. C to decompose the formic acid therein. However, at the elevated temperature involved in the use of this and certain other dehydrogenation catalysts disclosed in Russian Pat. No 57,862, substantial decomposition of the acetic acid also occurs. Selective oxidation both with and without added oxygen or air has been effected using a variety of catalysts such as platinum, osmium, iridium, palladium, ruthenium and rhodium supported on activated carbon or aluminum oxide and metal molybdates as described, for example, in U.S. Pat. Nos. 2,688,635, 2,913,492, 2,900,413, 3,196,176, 3,384,659 and 3,560,560. Other processes for selectively decomposing formic acid in mixtures containing formic acid and acetic acid have been developed which employ complex compounds of a noble metal of Group VIII such a platinum, osmium, rhodium or preferably ruthenium or iridium, these compounds being refluxed with the acid mixture as described in U.S. Pat. No. 3,488,383. In another method described in U.S. Pat. No. 3,459,707, the acid mixture is contacted in the liquid phase at temperatures from about 80.degree. to 140.degree. C with molecular oxygen in the presence of a catalyst comprising a soluble compound of a platinum group metal and a redox system and preferably also in the presence of a base. While such processes are effective, they are neither as simple nor as economical to operate as might be desired, particularly when only trace amounts of formic acid are present in the acetic acid to be treated. It is, accordingly, an object of the present invention to provide a relatively simple method for removing small amounts of formic acid from acetic acid which is readily adaptable for use in conjunction with the usual distillation techniques for purification of acetic acid.